Photosensitive photographic exposure timer



Nov. 9, 1965 R. A. CLAPP 3,217,167

PHOTOSENSITIVE PHOTOGRAPHIC EXPOSURE TIMER Filed Sept. 17, 1962 IN V ENTOR.

United States Patent 3 217,167 PHQTOSENSITIVE PfIOTgGI-IIC EXPGSURE TIMR Roy A. Clapp, 10522 Foley Blvd, Coon Rapids, Minn. Filed Sept. 17,1962, Ser. No. 223,911 4 Claims. (Q1. 250-207) The present inventionrelates to a timer used to control the exposure in photographicprinting, and in particular to such a timer employing a multiplier phototube.

The use of a conventional photo cell to provide a signal proportional tothe light transmitted by a negative, and using this signal to charge atiming capacitor to a voltage suficient to fire a trigger circuit whichterminated the photographic exposure, is well known in the art.

Unfortunately the conventional vacuum photo cell has a very low signaloutput and consequently the leakage currents inevitably present,together with grid emission of the trigger valve, prevent good linearitybetween the light transmission of the photographic negative and thetiming characteristics of systems where the conventional vacuum photocell is employed.

The use of a multiplier photo cell, while overcoming this disadvantage,introduces several problems.

The polarity of the signal output from a multiplier photo cell is suchthat either it must be inverted, as by a buffer tube, before beingapplied to charging a timing capacitor which is arranged to fire athyratron tube, or it must have its high voltage supply very wellisolated from and floating with respect to the trigger circuit powersupply. The use of two separate power supplies without a commonconnection makes it difiicult if not impossible to properly suppresstransients, a lack of stability in the operation which produces erraticexposures.

An object of the present invention is to provide a photographic timeremploying a multiplier photo cell and having a simplified circuitry withfewer components than has been required heretofore. This results in alower first cost and greater reliability.

Another object of the present invention is to provide a bias voltagesource for the trigger circuit from the regulated high voltage supply ofthe photo cell thus securing a reference voltage of great reliabilityand freedom from voltage fluctuation which is a cause of erraticoperation in photographic timers.

A further object of the present invention is to provide a system havinga common point of connection between the high voltage supply of thephoto cell and the main power supply of the trigger circuit.

Another object of the present invention is to provide a system in whichthe polarity of the photo cell signal being used to charge the timingcapacitor is of the correct polarity to fire the trigger circuit withoutthe intervention of inverter tubes or the like.

A further object of the present invention is to eliminate the need of avoltage divider in the trigger circuit power supply thereby reducing theamount of power required while at the same time eliminating the heatgenerated thereby.

Another object of this invention is to provide a means for biasing thetrigger valve while permitting the cathode of this valve to be connecteddirectly to the negative terminal of the trigger circuit power supply,which resistor acts to restrict the current through the thyratron andthe associated relay making it necessary to utilize a sensitive relayfor this purpose.

These objects and other advantages of this invention will be bestunderstood from the following description when read in conjunction withthe accompanying drawing.

The drawing is a schematic diagram of a photographic timer embodying thepresent invention. While in general this invention is specificallyconcerned with a combined 3,217,167 Patented Nov. 9, 1965 photo cell andtrigger circuit, suflicient associated apparatus has been shown toform'a complete and functioning timer.

The photo cell 1 1 is of the photo multiplier type, having a cathode 12,dynodes 13 through 21 and an anode 22, all enclosed in an evacuatedglass envelope 23. A regulated source of high voltage, schematicallyindicated as a battery 70, having a resistor in series therewith andhaving a negative terminal 82 and a positive terminal 83. Between theseterminals block 81 indicates schematically a voltage regulating device.A number of suitable devices incorporating glow discharge tubes or solidstate devices are well known. Further it should be understood that boththe high voltage power supply just described and the main power supplyfor the trigger circuit may readily be derived by transforming andrectifying commercial AC. power and in this case the regulation of thehigh voltage supply may be provided by a voltage regulating transformeror by electronic voltage regulators.

The negative terminal 82 is connected to the cathode 12 and the positiveterminal 83 connected to the anode 22; between these two elements isconnected a resistance string comprised of resistors 24 through 34 andserving to form a voltage divider so arranged as to provide a DC.potential starting from the cathode 12 progressively increasing to theanode 22. Thus, a resistor 24 is connected between the cathode 12 andthe dynode 13. Another resistor 25 is connected between the dynode 13and the dynode 14. Resistors 26, 27, 28, 29, 30, and 31 are likewiseconnected individually between the individual dynode stages 14 through20. However, resistor 32 is not directly connected between dynode 20 anddynode 21, instead one end of the resistor 32 is connected to plate 41of a capacitor 35 and the other plate of this capacitor is connected tothe dynode 21. Plate 41 is also connected to one end of the resistor 33and through 34 to the anode 22 and to the positive terminal of the powersupply. The power supply 70 would normally be of about 1250 volts whenthe photo cell 11 is a tube similar in type to the RCA 931A. The plates41 and 40 of capacitor 35, which acts as the timing capacitor, arenormally maintained in a shorted condition by a conductor 36 beingconnected to plate 41 and a conductor 37 being connected to plate 40,conductors 36 and 37 being shorted by the fixed contact 57 and movablecontact 56 of the relay 53. When in this condition each of the dynodestages has a voltage higher than the preceding dynode stage so that thephoto cell functions in a normal manner when exposed to light. Morespecifically, as light strikes the cathode 12 it causes a few electronsto be emitted, and these electrons are drawn over by the relativelyhigher potential to the first dynode stage 13. The impact of theseelectrons on the dynode ejects, through secondary emission, a largernumber of electrons which are drawn over to dynode 14 which isprogressively higher in potential and that this action is so continueduntil all the electrons produced by the cumu lative secondary emissionof the various dynodes are drawn over to the anode 22. In the course ofthis action a number of electrons are drawn from the dynode 20 to thedynode 21 and due to the multiplying action inherent in the tube, foreach of these electrons, a much larger number of electrons are ejectedfrom the dynode 21 and drawn over to the anode 22.

A trigger circuit comprising a thyratron tube 45 which has a heaterserving as a cathode 46, a grid 47, and an anode 48. The cathode 46 andgrid 47 constitute the voltage sensitive input to the trigger circuit,and as is well known, when the potential of the grid is substantiallynegative with respect to the cathode no current will flow through thetube when power is applied between the anode 48 and the cathode 46. Thiscondition is created by biasing the thyratron 45 in the following way.The heatercathode 46, energized by battery 49, is connected to thejunction of resistors 33 and 34 by means of conductor 39. The grid ofthis tube is connected to the plate 40 of condenser 35 by means ofconductor 37 and conductor 38. Conductor 37 connects this same plate tothe movable contact 56 of relay 53 and the plate 41 of condenser 35 isconnected by means of conductor 36 to the fixed contact point 57 ofrelay 53. Arm 56 and contact 57 are normally closed, thus maintainingcondenser 35 in a shorted condition so that there is no potentialdifference between the plates 40 and 41. The voltage drop of resistor 33as part of the voltage divider across the power source 70, serves tobias the thyratron 45.

The plate 48 of the thyratron tube 45 is connected by means of conductor60 to one side of a coil 50 of the relay 4'9. The other side of the coil50 is connected to a bus 65, connected to this bus 65 is one terminal ofan electro-mechanical shutter 58. The other terminal 67 is connected toa negative terminal of a main power supply shown in a detachedrelationship so that the triangular symbol G represents the negativeterminal of this power pack.

Also connected to the bus 65 is one terminal 61 of a push button 63 andthe other terminal of this push button 62 is connected to the positiveside of the power pack as denoted by the triangular symbol F. Alsoconnected to the terminal 65 is one side of a coil 71 of a relay 53, theother terminal of said coil being connected to the terminal G of themain power supply. Further the bus 65 extends to make a connection withthe relay 53, having movable contact 55 and a normally open contactpoint 54. A conductor 72 connects point 54 with point 52 of relay 49,the arm 51 of this relay is connected to the positive terminal F of themain power supply.

Having now described the arrangement and connections of the particularembodiment of this invention which is shown in FIGURE 1, the operationof this timer will now be described.

It is understood that a sample of the printing light is directed towardthe cathode 12. There are a number of well known methods to do this; forexample, a light refiected from the surface of the sensitized material,or beam splitter interposed between the negative and the printingmaterial, or light transmitted through the sensitized material are allpractical ways of providing a sample of the printing light.

To start the exposure, the push butt-on 63 is momentarily depressedconnecting the positive main power through contact points 62 and 61 tothe bus 65. The power thus energizes the electro-mechani-cal shutter 58and the shutter opens, starting the exposure. At the same time the coil71 of relay 53 is energized, the circuit being completed by the otherterminal of the coil which is connected to the negative terminal of themain power supply. As soon as the relay 53 is energized, arm 56 is drawnaway from the contact point 57 which is normally closed and thus thisparticular circuit is opened at the same time the contact arm 55 isdrawn upward and closes the contact with the point 54 which receivespositive power from the conductor 72 connected to point 52 of relay 49,the arm of which 51, is connected to the positive terminal of the mainpower supply. Thus, contacts 55 and 54 serve as a lock-in, maintainingthe relay 53 and the shutter 58 in an energized condition. The coil 50has one side connected to the bus 65 and the other side of this coil isconnected by conductor 60 to the plate 48 of the thyratron 45. Sincethis thyratron is arranged so that the cathode is at a higher potentialthan the grid, this tube is blocked and remains in a nonconductingstate; therefore, the relay 49 remains de-energized during the exposureperiod.

It will be remembered that the condenser 35 has been maintained in ashorted condition so that both plates are at a common potential prior tothe instant of the start of the exposure. With the opening of theshorting contacts 56 and 57 of relay 53 the short is removed across thiscondenser and it is now capable of changing potential by action ofchanging its state of charge. The plate 41 being connected to thevoltage divider associated with the high voltage power supply ismaintained at areference voltage dependent only upon the values of theresistors in said voltage divider. The plate 40, however, now begins tochange potential for the following reason. As electrons are conveyedfrom dynode 20 to dynode 21 they cause a larger quantity of electrons tobe drawn away from dynode 21 and toward the anode 22. 'If the quantityof electrons being drawn to anode 22 were identical with the quantity ofelectrons furnished by the dynode 20 there would of course, be no effecton the capacitor 35. However, since the multiplying action of thesecondary emission causes many more electrons to be drawn over and awayfrom the dynode 21 this deficit must be made up by the capacitor 35supplying electrons to the dynode 21. This emission of electrons fromthe capacitor plate 40 causes this plate to become charged in a positivedirection and since this plate is connected through conductors 37 and 38to the grid of the thyratron trigger tube, the trigger tube will fireafter a passage of time which is predetermined by the light intensityand the amount of bias initially supplied to the trigger tube by theresistor 33. As soon as the firing point of the thyratron 45 is reachedthe tube goes into conduction and closes the circuit across the relay 49causes the arm to lift breaking the connection between the main powersupply andcontact 52.

Power is now cut ofi" from the bus 65. Both relays are now free to openand the shutter 58 is de-energized at the same time so that it closes,terminating the photographic exposure. At the same time the contactpoints 56 and 57 are reclosed, discharging the timing capacitor 35 andplacing it in a condition for acceptance of the next printing cycle. Ofcourse, breaking the power supply to the bus 65 cuts off the power tothe thyratron 45, terminating its conduction.

-It will be noticed that the two power supplies, the main power pack andthe high voltage supply, are connected together through resistor 34.

There are several advantages to this. The cathode of the trigger circuitis connected directly to the negative of the main power supply so thatthe full voltage of this supply is across the thyratron and relay coilwithout any series resistor, which is normally required when the biasvoltage is supplied from the main power supply. There is no voltagedivider in this main power supply and hence no loss of power or heatdeveloped. On the other hand the bias voltage is supplied by theregulated high voltage supply, and since it is a requirement that thepower supplies for photo multiplier tubes be closely regulated, itfollows that the bias potential for the thyratron is also very closelyregulated, being in fact, a small percentage of the total fluctuationfor the entire power supply. Since this potential is applied through thegrid which does not draw current during the timing period, the power issubstantially nil and the normal voltage divider required for the .photomultiplier tube serves this additional function without added powerrequirement, and with no appreciable heat generation.

A very important advantage of the present invention lies in the factthat the two power supplies are tied together at a common point by meansof the resistor 34 as has been explained previously. This renders iteasy to suppress switching transients which would lead to erraticoperation, in a way which is completely impossible when the two powersystems are left floating with respect to one another. Capacitors may beshunted across resistor 33 and/ or resistor 34 for this purpose.

Since it is'common practice to integrate various program functions withthe automatic timer in a photographic printer, it is understood that thepresent invention may be applied in various ways in practice. Itherefore claim as my invention:

1. A photographic exposure timer comprising a trigger circuit powersupply, a voltage sensitive trigger valve, having cathode, grid andplate elements, serving to operate exposure terminating means, aphotocell voltage supply, a photocell of the multiplier type having acathode, anode and a plurality of dynodes, arranged to receive a sampleof the exposing light and to produce an electrical signal at one of saiddynodes as a function thereof, a tapped voltage divider connected acrosssaid photocell voltage supply for supplying biasing potentials to saiddynodes, a timing capacitor arranged to receive said electrical signal,one side of said capacitor being connected to the grid of the triggervalve and to said one dynode, the other side of the capacitor beingconnected to the voltage divider, the cathode of the trigger tube beingconnected to the voltage divider at a point nearer the positivepolarized end of the voltage divider than said capacitor connection.

2. A photographic exposure timer comprising a timing capacitor, atrigger circuit power supply, a voltage sensitive trigger valve, havingcathode, grid and plate elements, serving to operate exposureterminating means, a photocell voltage supply, a multiplier photocell,having cathode, anode and dynode elements, a tapped voltage dividerconnected across said photocell voltage supply with the negativepolarized end of said divider connected to the photocell cathode and thepositively polarized end connected to the anode, all but one of thephotocell dynodes being connected to its own tap along the divider, theremaining dynode being connected to one side of said timing capacitorand to the grid element of the trigger valve, the other side of saidtiming capacitor being connected to a tap on the voltage divider, thecathode of the trigger valve being connected to a tap on the voltagedivider at a point more positive than the tap connected to the timingcapacitor, switch-means normally closed connected across said timingcapacitor and arranged to open at the start of a timing cycle.

3. A photographic exposure timer comprising a photomultiplier tubehaving a cathode, anode and a series of dynodes therebetween, powersupply means for said photomultiplier tube including a voltage dividerhaving a plurality of resistance sections with taps connected to saiddynodes, a timing capacitor having one plate connected to the dynodenearest said anode and its other plate connected to one of said taps,means normally providing a shortcircuit across said capacitor, means forremoving said shortcircuit at the moment a photographic exposureinterval is to be initiated, and means connected to a point between thesaid dynode nearest said anode and said other plate of the capacitor forterminating said exposure interval when said other plate reaches apredetermined potential.

4. A photographic exposure timer comprising a photomultiplier tubehaving a cathode, anode and a series of dynodes therebetween, powersupply means for said photomultiplier tube including a voltage dividerconnected between said cathode and anode having tapped resistiveportions thereof connected to said dynodes and a further resistiveportion connected between said anode and ground, a thyratron having acathode, anode and grid, said cathode also being connected to ground, atiming capacitor connected between the dynode nearest said anode and oneof said tapped resistive portions, said grid also being connected tosaid nearest dynode whereby said thyratron is biased by said powersupply means for said photomultiplier tube, and means in circuit withsaid thyratron anode and cathode for terminating a photographicexposureinterval when said thyratron becomes conductive.

References Cited by the Examiner UNITED STATES PATENTS 2,503,165 4/50Meyer 250207 2,518,948 8/50 Simmon 250207 X 2,560,748 7/51 Silva 250207X 2,576,661 11/51 Wouters 250207 2,579,764 12/51 Schwennesen 250207 X2,615,079 10/52 Pardue et a1 250-207 X 2,810,078 10/57 Ballet al 250207X RALPH G. NILSON, Primary Examiner. FREDERICK M. STRADER, Examiner.

2. A PHOTOGRAPHIC EXPOSURE TIMER COMPRISING A TIMING CAPACITOR, ATRIGGER CIRCUIT POWER SUPPLY, A VOLTAGE SENSITIVE TRIGGER VALVE, HAVINGCATHODE, GRID AND PLATE ELEMENTS, SERVING TO OPERATE EXPOSURETERMINATING MEANS, A PHOTOCELL VOLTAGE SUPPLY, A MULTIPLIER PHOTOCELL,HAVING CATHODE, ANODE AND DYNODE ELEMENTS, A TAPPED VOLTAGE DIVIDERCONNECTED ACROSS SAID PHOTOCELL VOLTAGE SUPPLY WITH THE NEGATIVEPOLARIZED END OF SAID DIVIDER CONNECTED TO THE PHOTOCELL CATHODE AND THEPOSITIVELY POLARIZED END CONNECTED TO THE ANODE, ALL BUT ONE OF THEPHOTOCELL DYNODES BEING CONNECTED TO ITS OWN TAP ALONG THE DIVIDER, THEREMAINDER DYNODE BEING CONNECTED TO ONE SIDE OF SAID TIMING CAPACITORAND TO THE GRID ELEMENT TRIGGER VALVE, THE OTHER SIDE OF SAID TIMINGCAPACITOR BEING CONNECTED TO A TAP ON THE VOLTAGE DIVIDER, THE CATHODEOF THE TRIGGER VALVE BEING CONNECTED TO A TAP ON THE VOLTAGE DIVIDER ATA POINT MORE POSITIVE THAN THE TAP CONNECTED TO THE TIMING CAPACITOR,SWITCH-MEANS NORMALLY CLOSED CONNECTED ACROSS SAID TIMING CAPACITOR ANDARRANGED TO OPEN AT THE START OF A TIMING CYCLE.